1. Field of the Invention
This invention relates to fuel assemblies for nuclear reactors and, more particularly, to methods and apparatus for inserting and receiving fuel rods within grid plate spacer and support structures.
2. Summary of the Prior Art
In heterogenous nuclear reactors, nuclear fuel is frequently maintained separately from the moderator and coolant by encasing the fuel in thinwalled, cylindrical tubes known as fuel elements or rods. Groups of multiple fuel rods, commonly designated fuel assemblies, may be spaced in carefully designed arrays and laterally supported at several points along their longitudinal axes by grids of plates intersecting and interlocking in an egg-crate fashion to form a cellular lattice. One fuel rod is lodged in each of the cells thus formed in the grid structure. Generally, protrusions or the like, projecting from the surfaces of the portions of the plates bear against the outer surface of the fuel rod within a particular cell, serving to support the rod and restraining rod motion.
In order to preclude premature failure of the fuel rods, it is important to design the grid plate structure so that the protrusions do not mar the fuel rods during their insertion and removal. Hence, the prior art teaches the use of combinations of rigid and resilient protrusions, and external means which deflect the resilient protrusions to permit unhindered fuel rod insertion and removal.
The neutron absorption characteristics, mechanical strength, and corrosion resistance of the material used to form the grid plates are also important. Any absorption in the reactor of neutrons which do not cause further fission or the production of new fissionable material is defined as parasitic absorption. Neutron economy is the degree to which neutrons in the reactor are used for desired ends such as propagation of the chain reaction, converting fertile to fissionable material, or producing isotopes instead of being lost by parasitic absorption or leakage. In order to maximize neutron economy, it is desirable to minimize the volume of material used to construct the grid plate structure and to utilize materials having a low probability of neutron capture, as is measured by the neutron capture cross section of the material.
A balance must be struck between the often conflicting requirements of sufficient material to construct a grid plate which positively supports and spaces the fuel elements, which has sufficient resilience, and which has a low neutron capture cross section.
Zirconium and its alloys, chiefly in the form of zircaloy, are known to have a generally lower neutron cross section than any metal of comparable mechanical strength and corrosion resistance. However, as noted, it has been found necessary in the prior art to provide resilient protrusions to prevent undue damage to the fuel rod surfaces. Suitable spring materials, such as Inconelalloy 718, have relatively high neutron absorption. Inconel, a registered trademark of the International Nickel Company, Inc., has the following nominal composition (in percent):
Cr -- 15.0 PA1 Fe -- 6.75 PA1 Al -- 0.8 PA1 Ti -- 2.5 PA1 Cb -- 0.85 PA1 Mn -- 0.7 PA1 C -- 0.04 PA1 ni -- balance. Hence, apparent economic advantages can be realized in a grid plate design substantially utilizing zircaloy plates.
Thus, there exists a need to provide an efficient, economical apparatus and method for inserting fuel rods into cells of a grid plate lattice that utilize protrusions to positively space and support the fuel elements without marring the fuel element surfaces.
A versatile grid plate design, moreover, adaptable for supporting the tubes of vapor generators, shell and tube heat exchangers and the like, would offer further advantages.